$f(x) = 3x + 5$ for all real numbers. What is $f^{-1}(x)$, the inverse of $f(x)$ ? 2 4 6 8 \llap{-}4 \llap{-}6 \llap{-}8 2 4 6 8 \llap{-}4 \llap{-}6 \llap{-}8 $f(x)$ $f^{-1}(x)$
Answer: $y = f(x)$ , so solving for $x$ in terms of $y$ gives $x=f^{-1}(y)$ $f(x) = y = 3x+5$ $y-5 = 3x$ $\dfrac{y}{3}-\frac{5}{3} = x$ $x = \dfrac{y}{3}-\frac{5}{3}$ So we know: $f^{-1}(y) = \dfrac{y}{3}-\frac{5}{3}$ Rename $y$ to $x$ $f^{-1}(x) = \dfrac{x}{3}-\frac{5}{3}$ Notice that $f^{-1}(x)$ is just $f(x)$ reflected across the line $y=x$.